3L - If you have n options (26 letters) and you are choosing k times (3 positions), the total number of permutations is n^k. So, in this case, it's 26^3, which is: 26 * 26 * 26 =
17,576
3N - 000-999 ->
1000
4L - In this case, it's 26^4, which is 26 * 26 * 26 * 26 =
456,976
4L+N - For this one, n = 36 (26 letters + 10 numbers) and k = 4 (four positions), so the calculation is 36^4: 36 * 36 * 36 * 36 =
1,679,616
3L are all taken, same for 3N... 4Ls are still dropping but your best chance is in backordering auctions. Research LLLLs before buying anything, at this moment "chips" domains (random characters) are uncertain for short / medium term, at least in my opinion.
Good luck